There are many processes in the food and plastics industries where a measurement of the shear stress generated by a flowing fluid would provide valuable information for automated process control. As an example, in a reaction extrusion process, the shear stress generated on the inside wall of the extruder would provide a measurement of the material viscosity, and hence an indication of the state of reaction in the process. Such a liquid shear stress sensor for applications in a commercial extruder requires measurements at high shear stress (100 kPa), high pressure (5,000 psi), and high temperature (300.degree. C.).
A variety of techniques exist for the measurement of wall shear stress including the Preston tube, Stanton tube, Sublayer fence, thermal methods and direct measurement techniques. In a pressure sensing tube, such as the Preston tube, pressure is sensed as a function of the non-dimensional velocity profile. The velocity profile is rendered non-dimensional using the wall shear stress. Thus, the measured pressure can be directly related to the wall shear stress. This method, however, depends on knowledge of the velocity profile which can be assumed known only in a limited number of cases.
The usefulness of a direct measurement has motivated a variety of shear stress sensors known as floating element balances or floating element sensors. Such sensors include a plate positioned in an opening in the sample wall and supported on a pedestal. An electric coil or force is used to reposition the shearing plate to equilibrium. The current in the coil or the force used in recentering the plate serves as a measurement of the amount of average shear stress experienced by the element. However, this method usually does not allow for the measuring of fluctuating shear stress, and only measures relatively large shear stress. Further disadvantages in this approach have been problems with pressure gradient across the floating element, fluid flow through the gap area which produces apparent and erroneous forces, communication with ambient due to the large gap, and scale resolution.
One such floating element shear stress sensor is disclosed in U.S. Pat. No. 4,896,098 by Haritonidis et al., issued Jan. 23, 1990. That shear stress sensor employs a microdimensioned plate suspended above a substrate by microdimensioned arms or tethers. The microdimensions render an extremely small shear stress sensor which substantially reduces the problems of pressure gradient across the floating element (plate), gap flow and scale resolution. The sensor also enables the resolving of very small turbulent scales. Further, the plate is suspended at a height above the substrate which forms a very small passage or cavity between the plate and substrate. The dimensions of the passageway are so small that movement of the plate by forces due to vibration is heavily dampened by a viscous damping within the passageway. The dimensions of the plate and the damping effect of the passageway enable the microdimensioned sensor to be substantially insensitive to normal forces yet very sensitive to shear forces acting on it. Read out means which are also insensitive to vertical movement are incorporated in the sensor to provide an indication of sensed shear stress.
In the preferred embodiment of U.S. Pat. No. 4,896,098, the read out means employs a conducting layer associated with the sensor plate. The conducting layer is part of an integrated differential capacitance measuring circuit which produces the sensor read out. Such read out means are not suitable for measuring shear stress in elevated temperatures or in a liquid environment. Thus, the floating element shear stress sensor of U.S. Pat. No. 4,896,098 is limited in application depending on the read out means employed.
Other shear stress sensor methods only measure forces in an air fluid or a water fluid but not both. Some methods are dependent on gravity and thus only work on a horizontal target surface. Furthermore most methods are hard to install, may disturb the flow of the fluid, and are expensive.